A wholly unreinforced back pack has the advantage of extremely light weight and can be extremely comfortable to wear for highly athletic activities like mountain climbing or cross-country skiing. Nonetheless, if it is not packed extremely carefully it can be a substantial hindrance to the wearer, as a hard object in the pack can bump the wearer's back painfully or the entire pack can be imbalanced and wobble when the wearer is moving rapidly.
A pack with a built in front frame avoids many of these problems and allows substantially more to be carried without difficulty. Such packs are, however, often fairly heavy even when empty. In addition they are not comfortable when the wearer must bend a great deal. An attempt to cure this by making the reinforcement semirigid so it can bend somewhat is a compromise that is often an uncomfortable fit and that swings excessively if the wearer is moving rapidly.